This invention relates to an improved process for polymerization of ethylene. More specifically, improved efficiency is attained by use of membranes to recover ethylene from vent gases.
Processes for manufacture of polyethylene or other ethylene polymers generally do not achieve total conversion of ethylene. Conventionally, ethylene is separated from the ethylene polymer by flash devolatilization. The recovered ethylene can be recycled if of suitable purity. Over time ethane, methane, diluents and other undesirable impurities accumulate in the vent gas, i.e., the mixture of unconverted gaseous reactants, impurities and by-products resulting from the polymerization process. In catalyzed polymerizations these impurities can reduce catalyst activity. Cryogenic separations for recovery of ethylene are relatively expensive. Likewise adsorbent systems are costly in capital, energy and operating costs. Consequently, frequently the impure ethylene is limited to uses where it is less valuable than as a monomer.
U.S. Pat. No. 3,758,603 describes the use of a liquid barrier on a semi-permeable membrane to separate ethylene from ethane and methane. The liquid barrier contains complex forming metal ions. However, these liquid barriers have not proven very durable in commercial operation.
A method of separating ethylene which is simple, durable and cost-effective is desirable. Applicants have now found that normally solid semi-permeable membranes which traditionally have been viewed to afford inadequate separation of ethylene and ethane can surprisingly enhance the ethylene efficiency of polymerization processes.